To search for primeval galaxies, already
Elston et al. (1988)
had identified extremely red objects that are conspicuous only in the
near-infrared, and have R - K colors larger than 5. The ERO class
could include several categories of objects: essentially
old stellar populations at high redshift, or high-z dust-reddened
starbursts
(Hu & Ridgway 1994;
Cowie et al. 1996).
Maybe 10% of the submm sources could be EROs
(Smail et al. 1999).
A proto-typical ERO at z = 1.44
(Dey et al. 1999)
has been detected in submm continuum
(Cimatti et al. 1998),
and has been found to be an ultraluminous starburst shrouded by dust,
with a star formation rate of 200-500
M per yr.
The surface density of such EROs at K < 20 and
color R - K > 6 is about 500 per square degree,
and for R - K > 7, about 50 per square degree
(which is comparable to the number of QSO with B < 21.5,
Andreani et al. 1999).
At least for the few objects known, the AGN cannot
be the source of the huge luminosity, given the FIR/radio ratio.